1. Field of the Invention
This invention relates to a steering system for a vehicle, and more particularly to a four-wheel steering system for a vehicle in which the rear wheels are arranged to be turned in response to the turning of the front wheels when the steering wheel is turned.
2. Description of the Prior Art
In U.S. Pat. No. 4,418,780, there is disclosed a four-wheel steering system for a vehicle in which a front wheel turning mechanism for turning front wheels and a rear wheel turning mechanism for turning rear wheels are provided, and the rear wheel turning angle characteristics with respect to the front wheel turning angle is changed according to the vehicle speed so that the rear wheel turning mechanism is controlled to nullify the side slip angle of the vehicle body according to the vehicle speed and the front wheel turning angle.
In accordance with the four-wheel steering system having such rear wheel turning angle characteristics that nullify the side slip angle of the vehicle body, the rear wheels are turned in the direction opposite to the turning direction of the front wheel (reverse phase) during low speed travel of the vehicle, thereby improving the cornering performance to reduce the minimum turning radius of the vehicle, while the rear wheels are turned in the same direction as the front wheels (same phase) during high speed travel of the vehicle, thereby improving the steering performance to facilitate smooth lane changing.
The four-wheel steering system is aimed at obtaining an effect whereby the vehicle travels in a direction tangential to the desired turning circle (i.e., the lateral acceleration of the vehicle conforms with the cornering motion) so that the driver can accurately predict the traveling direction of the vehicle and can drive the vehicle with a natural feeling.
However, in the actual steering performance of the vehicle, the steering characteristics that nullify the side slip angle are not always considered, by the driver, to provide good steering performance and high stability since the response characteristics of the lateral acceleration to operation of the steering wheel differs from that of the yaw rate.